Darwin’s Finches Struggle To Woo Females With Song Thanks To Beak-Deforming Parasite

Parasites are nasty little bastards. The freeloaders survive by siphoning off the vital nutrients our bodies depend on, making us feel tired and sick. But, bizarrely, they can also impact sex.

The burden of a parasitic infection can alter how attractive an animal is to the opposite sex; male greenfinches and cichlid fish swap their snazzy feathers and scales for drabber coloration, for example.

Now, a study published in the Proceeding of the Royal Society B has looked at the phenomenon in the Galapagos finches made famous by Darwin, finding that males infected with parasitic fly larvae sing the songs they use to woo females out of tune, which impacts their chances of mating.

Philornis downsiis a species of fly that is believed to have accidentally been introduced to the wildlife haven of the Galapagos some 60 years ago. Its larvae survive by feeding on the blood and tissues of developing finches, causing about 55 percent of chicks to die before they fledge and leading to malformed beaks and enlarged nostrils in those that survive. The fly has been identified as the biggest threat to the survival of all land birds on the Galapagos Islands.

“An individual that survives consumption by P. downsi can be left with such severe naris malformation that the upper nasal cavity is missing and the beak is laterally open from one side to the other,” the researchers write. “In less extreme cases, the naris is enlarged and/or plugged up, often with a residual larva that failed to emerge. Philornis downsi has changed the beak of the finch.”

A team of scientists led by Flinders University wanted to determine how this might impact the birds’ mating songs. They analyzed the beaks and calls of two species of Galapagos tree finch, the small tree finch (Camarhynchus parvulus) and the medium tree finch (Camarhynchus pauper). They also looked at tree finch hybrids, birds produced by interspecies mingling.

The researchers found that males belonging to both C. parvulus and C. pauper sang songs with more vocal deviations and lower maximum frequencies when their nostrils were enlarged, although the same was not true for hybrid birds. In the two species, more vocal deviations mean fewer impressed females, so infected males take longer to find a mate. Meanwhile, C. pauper males normally sing songs specific to their species, but infected birds fail to do this, singing tunes indistinguishable from those of other species.

A lack of species-specific songs can lead to an increase in hybridization, as birds from different species confuse each other for members of their own. The researchers note, therefore, that parasitic infections can alter mating patterns and change species trajectories, leading to higher numbers of hybrid animals. They suggest that this could even lead to “reverse speciation”, where two individual species blur back into one.

The impact the parasite is having on how the birds mate “should be of particular cause for concern for the fate of Darwin’s finches,” the team warns. At present, the medium tree finch is listed as critically endangered.